Xanthan is an anionic extracellular polysaccharide produced by Xanthomonas campestris NRRL B-1459. The high viscosity of this polymer solution is relatively insensitive to temperature, ionic strength, shear and pH. For this reason, xanthan finds commercial use as a viscosity-enhancing agent for aqueous solutions. In particular, xanthan is used for enhanced oil recovery. The primary structure of xanthan was established by Jansson et al, 1975, "Structure of extracellular polysaccharide from Xanthomonas campestris" Carbohydr. Res. 45, 274-282. It consists of a main chain of .beta.-1,4-linked D-glucose units, as in cellulose, but with a three sugar side chain attached to alternate glucose residues. Pyruvic acetal, i.e., 4,6-0-(1-carboxyethylidene) substituents are on the terminal d-mannosyl residues of some of these side chains. The molecular weight of xanthan varies from 3.times.10.sup.6 to 15.times.10.sup.6 daltons, depending on the methods and conditions used in the determination of molecular weight.
Xanthan is inert to the attack by microbes or available enzymes, see, e.g., Kelco, Company "Xanthan Gum" Second edition, p. 9, Kelco Co., Clark, N.J. Rinaldo and Milas, 1980, "Enzymic hydrolysis of the bacterial polysaccharide xanthan by cellulase", Int. J. Biol. Macromol. 2, 45-48, were the first to show partial hydrolysis of xanthan by cellulase only in the absence of salt where xanthan is in the unordered conformation. More recently, Cadmus et al (1982) "Biodegradation of xanthan gum by Bacillus sp", Appl. Environ. Microbiol., 44, 5-11, reported the biodegradation of xanthan by a Bacillus sp. in the presence of salt. The xanthanase they obtained was a mixture of enzymes that attacked all of the side chain linkages in the xanthan molecule, including the one involving .beta.-1,3-linkage of acetylated mannose to the glucosidic backbone. They found no endo cellulase type of activity in their cultures. Sutherland, (1982), "An enzyme system hydrolyzing the polysaccharides of Xanthomonas species", J. Appl. Bacteriol. 53, 385-393, described an enzyme system hydrolyzing the polysaccharides of Xanthomonas species. The .beta.-glucanohydrolase hydrolyzed both .beta.-1,3- and .beta.-1,4-linked polymers with side-chains or other substituents. Both Cadmus et al and Sutherland deal with mixed cultures to obtain the best activity to degrade xanthan.
In recent enhanced oil recovery field test with xanthan as the viscosity controlling agent, xanthan was found to be degraded by microbial (enzymic) activity. Since then, the biodegradation of xanthan and methods for its prevention have become important research areas. Partial degradation of xanthan into polymers with different molecular weights is also important for more precise investigation of xanthan conformational properties.
The present invention includes a method for degrading the xanthan molecule using a mixed culture, which utilizes xanthan as its carbon source in the presence of salt. The invention also includes the extracellular enzyme(s) produced by the culture which degrade the xanthan molecule. In addition, the invention includes a novel depolymerase which breaks the endo .beta.-1,4-glucosidic linkage of xanthan molecules.